0
Till now, we have considered the motion of a single particle or a body considered as a particle represented as a point mass (practically no size) and various laws were applied on them. But ant real body which we see has a finite size. In dealing with the motion of these extended bodies i.e. bodies of finite size, the particle model is inadequate.
An extended body is nothing but a system of particles. E.g. if a ball is tossed into air without much spin, its motion is simple- it follows a parabolic path and the ball can be treated as a particle. However, if we flip a baseball bat into air, its motion is more complicated because every part of the bat moves differently, along paths of many different shapes. Therefore, it can not be represented as a particle. Instead it is a system of particles each of which follow its own path through the air. However, the bat has one special point- the centre of mass- that does move in a simple parabolic path. The other parts of the bat move around the centre of mass.Thus the motion of centre of mass of a system of particles is useful in understanding the motion of extended bodies. Many of the problems with extended bodies can be resolved by considering them a rigid body.
Rigid Body: Bodies with a perfectly definite and unchanging shape i.e. distance between all pairs of particles remain unaltered.
Though, no real body is truly rigid because they deform under influence of force (limit of force, though, is different for different bodies), but in case the deformation is negligible, these can be considered as rigid bodies. E.g. wheel, steel beams, planets etc.
Centre of Mass
Centre of mass of a system of particles is defined so as to predict the possible motion of the system. It may be defined as:
The centre of mass of a system of particles is the point that moves as though all the mass of the system is concentrated at that point and all external forces are applied at there.
